The parametric loudspeaker is an electroacoustic system that operates by producing an ultrasonic carrier frequency, for example 40 kHz, that is then modulated by an audio input signal. The modulation shifts the audio frequency up to the frequency of the carrier plus the audio frequency. This upshifted frequency (f1) interacts with the carrier frequency (f2) thus generating an audible reproduction of the audio input signal by driving the air to non-linearity which produces the audible signal of interest (f1−f2) plus other components (such as f1+f2). The ultrasonic upper frequency requirement of a parametric system is typically at least 60 kHz because this allows 20 kHz of the audio signal to be modulated on top of the 40 kHz carrier signal.
Historically, the use of parametric loudspeakers has been limited. This is partially due to their general inefficiency because the sound output is based on a second order effect of the demodulation of ultrasonic sound waves in the air into audible sound. This second order effect needs a greater amount of power to drive the system and deliver the audio output.
Parametric power delivery systems also have further reduced efficiency because the parametric system requires a continuous carrier frequency output At full audio output, the carrier frequency is operated at a constant ¼ power output level, which causes high power dissipation in the amplifier. Even at lower audio levels or during a break in the music the carrier signal must be driven at high constant power levels.
Further, most parametric loudspeaker transducers exhibit highly reactive loads. In the prior art, parametric transducers are driven using a conventional linear power amplifier to directly drive the transducer, and they require very large power amplifiers that dissipate significant power and heat in the output stage.
Due to the high continuous power levels that can be required, the transducers that work best for parametric or ultrasonic loudspeakers tend to have dominantly reactive (capacitive and/or inductive) characteristics. This is in contrast to conventional electromagnetic speakers which tend to have a dominant resistive characteristic. One of the reasons for using reactive speakers in a parametric system is that the high average level of the carrier frequency can cause high thermal dissipation in the resistive element of any transducer. A purely reactive transducer dissipates very little heat in the device itself because of the reactive load it provides to the amplifiers. Correspondingly, the output stage of the power amplifier (particularly a linear amplifier) coupled to a reactive transducer or speaker has significant thermal losses. These losses are caused because the power amplifier must amplify highly reactive charging currents when driving the reactive load directly. The problem is particularly detrimental at the frequencies of greatest output, such as the carrier frequency and frequencies associated with lowest audio frequencies to be reproduced.
A related major issue with prior art parametric loudspeakers is that the reactive load transducers require significant reactive charging power. In turn, that power requirement has forced the use of much higher output power amplifiers to supply this wasteful power.
Prior art parametric loudspeakers have used what is commonly known in the art as a linear or Class B amplifier topology which reaches maximum efficiency at full power and is at its most thermally inefficient mode at the ¼ power level or the equivalent half voltage level. As an example, a 100 watt Class B amplifier when operating at ¼ power may dissipate 50 watts into wasted heat while outputting only 25 “useful” watts to the load. This is both an inefficient waste of power and a costly system to build because it can require extensive cooling systems.
A serious contributor to the inefficiency of a linear power amplifier in a parametric system is the fact that the common transducer type used in parametric loudspeakers has a reactive impedance that must be driven by the power amplifier. It is well known that linear amplifiers have a significant reduction in efficiency and increase in heat when driving a reactive load. Accordingly, it is desirable to provide a system which would allow a more efficient use of amplifier power in a parametric speaker system.